Makefile: fixing typo …
common: keep /dev mounted for yum

yum which called by clear_packages_cache() depends on /dev to do the
package clean up.
That's why we must call call clean_mount_points() after clear_packages_cache()
package: get ride of chrooted_yum()

chroot_yum() was more or less a hack to work around the /dev
mount/umonut mess. We don't need it anymore.

I tested this changed by build “base” and “mysql” successfully.